add aux input that doesn't need cassette in holder? 2000 Mercury Sable

I have a 2000 Sable with a radio that has built in cassette. Although I have used the audio cassette adapter for years, I've been wondering if a
good aux input modification is now available, and something I could do myself at minimal cost? The mods I keep seeing tap into the radio wiring harness speaker wires and they seem ok, but you either have to have a cassette playing and/or CD playing to work. If I keep having to have a cassette playing, I might as well stick with the cassette adapter. The cassette adapter has done pretty well overall, but as it ages the cassette head and head in the adapter start to misalign resulting in audio or frequency loss. If a non-cassette using aux mod has now come along, please share.
Thank you, JBI
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